The order comprises 100 of the new Boeing 737 Max 9 planes and 50 Next Generation 737-900ER aircraft. Boeing said the deal meant it had now received more than 10,000 orders overall for aircraft from the 737 family.

The order was announced in Chicago, corporate home for both companies, and is a major victory for the US firm against arch-rival Airbus.

Boeing said the 737 was the "undisputed best-selling jetliner in the world". It said the Next Generation 737 was "the most fuel-efficient and reliable" single-aisle plane in the market.

The deal comes after Airbus advertised a potential $16.9bn for a total of 115 air-crafts at the Farnborough Airshow in the UK. Airbus's total was way down on last year's record tally at the Paris Airshow of $72bn.

"United and Boeing share a rich history together and we are delighted United has chosen the 737 for its future fleet, renewing our partnership for decades to come," said Ray Conner, chief executive of Boeing Commercial Airplanes.

Jeff Smisek, chief executive of United, said: "We look forward to offering our customers the modern features and reliability of new Boeing airplanes, while also making our fleet more fuel-efficient and environmentally friendly."